More than 10,000 athletes will compete in the 2012 Summer Games, and nearly half of them — including every medal winner — will be tested for 240 banned substances under the most aggressive anti-doping campaign in Olympics history.

Collecting and analyzing those blood and urine samples, some 6,250 in all, will fall to a team of 1,000 people working in a nondescript laboratory north of London. The lab, provided by the pharmaceutical giant GlaxoSmithKline and run by King’s College London, will run 24/7 as it processes as many as 400 samples daily.

The only thing more aggressive than the unprecedented number of tests — 4,770 samples were analyzed during the 2008 Games in Beijing, and 3,600 in Athens — is the timeline for conducting them. London 2012 organizers started the testing as athletes began arriving last week and hope to complete nearly 40 percent of the tests before the Games officially begin Friday. The athletes are subject to testing at any time, in any place. The top five finishers in every event, and two others selected at random, will be tested. And every sample will be held for eight years to allow for retroactive testing as new tools and techniques become available.

“The challenge in 2012 is that athletes are using substances that mimic natural substances,” says Dr. Matthew Fedoruk, the science director of the U.S. Anti-Doping Agency, and a veteran of the anti-doping efforts of the 2010 Winter Games in Vancouver.

The challenge he’s referring to lies in the fact our bodies produce EPO, HGH and testosterone. Detecting what occurs naturally in the body and what has been artificially augmented is difficult if you test only for the substance in question. In many cases, athletes can boost, say, their EPO level by taking FDA-approved drugs developed to treat health problems like anemia.

The never-ending battle against performance-enhancing drugs protects the integrity of the Olympic Games as much as the health of athletes. Athens was a high-water mark for Olympic doping, with 26 cases, more than twice the previous record of 12 seen in Los Angeles in 1984. We may see still more; recent retesting of samples collected in Athens revealed five more suspected positive results.

Beijing saw 14 positive tests among athletes and six among horses in equestrian events. Later retests of samples identified five additional athletes who’d used a drug called CERA.

The fight against doping started with the 1968 Games, and today the job falls to the World Anti-Doping Agency, founded in 1999, and the U.S. Anti-Doping Agency, founded the following year. The technology to catch drug cheats has advanced rapidly in the past dozen years, with great strides made even since Beijing. Scientists have more sensitive tests and tools than ever to detect some 240 banned substances and procedures, from steroids and growth hormones to experimental drugs not yet on the market.

Still, it is often a case of two steps forward, one step back. Each new test — for erythropoietin(commonly known as EPO) in 2000, for human growth hormone in 2004, and CERA in 2008 — causes the dopers to change tactics in what has become the cat and mouse game of the Games.

But the doping police have a new tool: the biological passport. It is the latest, most ambitious and most effective development for catching (and clearing) athletes suspected of using prohibited substances. The passport, which tracks an athlete’s blood profile over time, doesn’t necessarily detect doping, but rather the physiological changes it causes. Those changes can indicate doping and lead to sanctions. What makes the passport especially attractive is it allows authorities to take full advantage of the eight-year statute of limitations so they can more effectively track athletes over time and revisit samples as new technology and techniques warrant.

“It’s a big step forward for anti-doping,” Fedoruk said. “It’s a move from the traditional toxicological approach to a forensic and medical approach so that we can follow up.”

The biological passport is a longitudinal evaluation that tracks certain biomarkers, like hemoglobin orreticulocytes that rise of fall during blood-doping regimens. These markers are monitored over time, and inconsistent or unexplained changes can flag someone for a more targeted test and possibly a sanction for doping. That’s key, because athletes inclined to use performance-enhancing drugs aren’t likely to do so during the Games. They’re far more likely to juice during intense training and in the weeks before major events.

Another key development since Beijing is a test for for CERA, or continuous erythropoietin receptor activator, a variant of EPO. It was developed to treat anemia caused by kidney disease. Authorities have since used the test to analyze samples collected during the 2008 Games, retroactively catching a handful of users including 1,500 meter gold medalist Rashid Ramzi of Bahrain, who was stripped of his title.

The 2012 Summer Games will see the adoption of a new blood test for human growth hormone that relies on indirect markers to reveal doping. This new test looks at other markers that are affected when HGH is used, extending the window of detection for HGH use by three weeks. This is a big step forward, because although HGH is most effective when used regularly, it doesn’t remain in the body very long, making detection tricky with other blood tests.

Testosterone also is on the forefront of current research involving the biological passport. Like EPO and human growth hormone, it can be difficult knowing what’s natural and what’s administered. A test, called the IRMS (Isotope Ratio Mass Spectrometry) method, looks at the C13/C12 (carbon) ratio of testosterone, and can determine whether someone took testosterone or produced it naturally. It’s well established that pharmaceutical testosterone lowers the delta C13 value and therefore can be detected in the test.

The biological passport is important in sensitive, sophisticated and expensive tests like this, which are only used only when there is strong suspicion that someone’s on the juice. Scientists can also look at what’s called the t-epi ratio, which — like hemoglobin concentration — remains mostly constant throughout life. The T/E ratio is one parameter labs look for, because if you take testosterone, your T/E will go up, and an increase will raise a red flag.

“If someone goes from 1 to 3, we might want to test that one,” said Dr. Daniel Eichner, lead researcher at Sports Medicine Research Labs, told Wired. The lab is one two in the United States used by WADA, and is the descendant of the lab that tested for the 2002 Winter Games. “This wasn’t done five years ago. You could use testosterone in a sophisticated manner and not be caught.”

In another advancement since Beijing, the major pharmaceutical companies are working with anti-doping agencies by alerting them to experimental drugs that could have performance-enhancing side-effects. This allows WADA to develop tests before the drug is even on the market.

One potential doping method that Eichner and others find concerning are experimental drugs containing growth hormone-releasing peptides. HGH in this instance is not administered but instead the peptides taken stimulate growth hormone secretion, making it even harder to detect. To make matters worse, many of these HGH-releasing peptides are not yet FDA approved and there are currently many versions available on the black market.

What’s next is anyone’s guess, because it seems the dopers are always ahead of the curve. One idea getting a lot of attention lately is gene doping — genetically manipulating cells to produce more EPO, for example, or manipulating the cells that regulate muscle mass. Such alterations could aid those with genetic diseases, but it isn’t difficult to imagine athletes using such techniques to boost performance. Fedoruk isn’t convinced such a possibility is, at this point, anything more than a theory, but neither will he rule it out.

Still, one thing is certain. In the cat-and-mouse game between dopers and doctors, the dopers will remain a step ahead.

“Athletes adjust doping practices based on the technology that we bring out in the labs,” Eichner said. “We make an improvement, they make an adjustment, and we go from there.”